1. Field of the Invention
The present invention relates to an improvement of a control apparatus for a variable voltage/variable frequency inverter controlled on the pulse width modulation basis, and particularly to an improved control apparatus for an inverter for feeding an induction motor, which is controlled to produce constant torque.
2. Background of the Related Art
It is known to control an induction motor by means of an inverter of a variable voltage/variable frequency (VVVF) type, which is operated on a pulse width modulation (PWM) basis, so as to produce constant output torque. For the PWM control of an inverter as mentioned above, there is often adopted a so-called sinusoidal wave modulation method. In such a modulation method, control signals for semiconductor switching elements, such as transistors or gate turn-off thyristors, of an inverter are produced by comparing a sinusoidal wave modulation signal and a triangular wave carrier signal, both of which are synchronized with each other.
An output AC voltage of such an inverter is controlled by varying a modulation factor, which is a ratio of the amplitude of a modulation signal to that of a carrier signal, and its frequency is controlled by varying the frequency of the modulation signal. Further, in the following, the frequency of an output AC voltage of an inverter will be simply called an output frequency. It is to be noted here that a maximum value of a modulation factor is limited by a minimum allowable turn-off time of semiconductor switching elements used in an inverter, although it will be described in detail later.
On the other hand, for the purpose of performing the constant torque control of an induction motor, an inverter must be controlled in such a manner that a ratio of an output AC voltage V thereof to its output frequency f is maintained at a required constant value; namely, the inverter is controlled in accordance with a predetermined V/f ratio characteristic . Moreover, it is usually carried out to change a number of voltage pulses within one cycle of the output AC voltage of a PWM-controlled inverter in accordance with a predetermined range of the output frequency, in order to reduce the amplitude of a ripple component included in an output current due to the PWM control. The change of the number of voltage pulses can be achieved by varying a ratio of the frequency of a carrier signal to that of a modulation signal.
In such a PWM-controlled inverter, the output AC voltage thereof is controlled by varying a modulation factor, while maintaining a number of voltage pulses at a desired value over a predetermined range of the output frequency. The operation of an inverter under a certain number of voltage pulses is identified by the number of voltage pulses, which is called "mode". For example, the operational mode of an inverter is changed form "9 pulse mode" through "5 pulse mode" and "3 pulse mode" and, finally, to "single pulse mode" with the increase of the output frequency.
Conventionally, the change of a pulse mode is carried out in accordance with reference frequencies fixedly provided in advance. Namely, when an output frequency becomes equal to the corresponding reference frequencies, a pulse mode is changed between a 9 pulse mode and a 5 pulse mode, between a 5 pulse mode and a 3 pulse mode, or between a 3 pulse mode and a single pulse mode, respectively. It is recognized that the reference frequencies as mentioned above are so selected that the amplitude of the ripple component of an output current is made as small as possible. In such a conventional control apparatus, however, there was a drawback as follows.
When a pulse mode is changed, there occurs a peak value in an output current of an inverter. If the voltage of a DC voltage source for an inverter is high, the aforesaid peak value in the output current becomes large accordingly, which may exceed the limit of a commutateable current of semiconductor switching elements of the inverter. On the contrary, if the DC source voltage is low, a non-conductive period of the switching elements is made very small, which may be smaller than a minimum allowable turn-off time. This causes the commutation failure in the inverter. For the purpose of eliminating the above mentioned drawback, there has been proposed a control apparatus as disclosed in the Japanese Patent Laid-open No. JP-A-57/129170 (published Aug. 11, 1982).
According to the prior art, in a PWM-controlled VVVF inverter, a present modulation factor is ascertained in every pulse mode by the calculation on the basis of the voltage of a DC voltage source for the inverter and an output frequency of the inverter, and the change of a pulse mode is carried out, when it is discriminated that the ascertained modulation factor reaches a maximum value thereof as determined on the basis of a present pulse mode and an output frequency at that time. In this manner, the prior art control apparatus can cope with the variation of the voltage of a DC voltage source for an inverter.
Actually, however, there are often cases, where a PWM-controlled VVVF inverter must be controlled in accordance with a different V/f ratio characteristic. In such cases, the prior art requires a new pulse mode change characteristic, which is suitable for a new V/f ratio characteristic. Therefore, the prior art control apparatus lacks the flexibility and was not suited for the wide use in various V/f ratio characteristics.